In a color cathode ray tube, in order to achieve a balance in the apparent brightness of different colors and correct rendering of secondary colors, it is necessary to compensate for variations in phosphor efficiency and transconductance between the three guns. The book entitled "Colour Television Theory" by G. H. Hutson (McGraw-Hill, London, 1971) describes, at pages 82 to 85, the adjustment of a color television to maintain a color balance for different contrast levels of a gray scale image. Firstly the lowlights are adjusted by bringing the cut-off points for all three guns into registry. This is done by adjusting the screen (the first anode) voltages of the three guns. Next, the highlights are adjusted to compensate for the slightly different gun slopes and the different phosphor efficiencies. This is done by varying the luminance signal drive to the cathodes of the three guns.
British Pat. No. 1,456,439 describes a color television receiver incorporating an in-line cathode ray tube rather than the older delta type. It is pointed out that in the kind of in-line tube to which the specification relates, there is a common first control grid and a common screen grid so that there is no provision for separate adjustment of the red, green and blue screen potentials to bring their cut-off points into registry. The specification describes in detail a control circuit for applying individually adjustable D.C. bias to the respective cathodes.
Neither the aforementioned book nor the aforementioned specification describe how the color balance is maintained for different settings of the brightness control of the receiver, it being assumed that once the balance has been set for different contrast levels at one setting of the brightness control, it will remain in balance for other settings.